In 2010, we published the first description of a genetically unique strain of Lassa virus isolated from Mastomys natalensis captured in Mali, an area previously thought to be free of Lassa virus. Over the last eight years we have continued to conduct field studies mapping the geographic distribution of Lassa virus in peridomestic settings across sub-Saharan Mali. With a better understanding of the regions endemic for this virus in Mali, the Lassa program has transitioned into studies aimed at defining the infection dynamics and transmissibility of Lassa in the natural rodent hosts. For this we have established a colony of Mastomys natalensis at the Rocky Mountain Laboratories. Several studies have shown that the animals can be persistently infected with different Lassa strains without showing obvious clinical signs but virus shedding through different routes (Safronetz, Rosenke, unpublished). We have also investigated the possibility of the Mastomys as a host for a spirochete Borrelia Crocidurae, a cause of relapsing fever. We found that the these animals support a long term infection and may in fact be an unreported host species for this disease causing bacterium (Rosenke et al., manuscript under preparation). Thus, this colony provides a unique tool for future Lassa virus infection studies in the natural host. More recently, we have started projects to define the incidence rates of Lassa virus infection in humans in southern Mali. Two human serosurvey studies were conducted at three sites; IgG seroprevalence was found at all sites ranging from 16-44%. Interestingly, in the follow-up study we identified an annual infection rate of 6.3%. As there are no clinical reports of Lassa fever in Mali, this indicates that asymptomatic and mild infections may frequently occur. Experimental studies using a Lassa isolate from Mali, designated Soromba, support this hypothesis as Soromba is less virulent in animal models and presents with an atypical respiratory disease in cynomolgus macaques (Safronetz et al., J Infect Dis 2013). A new two-part human study covering a larger geographic area is approved and will start in November/December 2019. We will examine a cross-section of the population as a whole looking for Lassa virus prevalence and collect clinical samples from hospitals to determine the prevalence of Lassa Fever. This study will confirm and expand upon our previous results and identify human infections to determine the public health importance of Lassa virus infections in Mali. A live-attenuated recombinant vaccine platform expressing the Lassa virus glycoproteins (rVSV-Lassa) showed efficacy against several Lassa isolates including Lassa-Soromba. This vaccine has been GMP-produced and the GMP lot is currently being sterility tested. A future phase I clinical trial is under preparation. More recently, we have evaluated the efficacy of ribavirin and favipiravir as antivirals against Lassa virus in two animal models (guinea pig and nonhuman primate) with favipiravir being superior over ribavirin (Safronetz et al., Sci Rep 2015; Rosenke et al., Emerg Infect Dis 2018). The Mali field program has expanded to include other zoonotic viruses including surveillance for additional arenaviruses (Lassa, Lujo), bunyaviruses (hantaviruses, Crimean-Congo hemorrhagic fever virus (CCHFV), Rift Valley fever virus), flaviviruses (Dengue, Zika), and coronavirus (MERS-CoV). We have started to also test serum/blood from humans and livestock and wildlife species to determine the prevalence and importance of zoonotic pathogens for animal and public health in Mali. As of today, we could demonstrate MERS-CoV infections in Malian camels and high CCHFV infection rates in Malian cattle. For humans, we could establish serologic evidence for flavivirus (Dengue, Zika), bunyaviruses (hantavirus), and togaviruses (Chikungunya) infections as well as infections with Leptospira spp.; all contributing to human illness in Mali. These results implying that several of these zoonotic pathogens are widely distributed yet underreported throughout Mali (Safronetz et al., Emerg Infect Dis 2016; Falzarano et al., One Health 2017; Maiga et al., Am J Trop Med Hyg 2017; Subudhi et al., Am J Trop Med Hyg 2018; This program also includes education and training of young Malian scientists. Ousmane Maiga received his Ph.D. in April 2019.